TL;DR: POST-QUANTUM SECURITY → PERMANENT PRIVACY blockchain data is stored forever. that means your encrypted onchain data needs to stay encrypted forever if the encryption breaks, all your transactions, from your salary to your trading history, become visible

adversaries can download encrypted blockchain data today and wait for encryption to break. once quantum computers come online, this will be a real problem, exposing sensitive transactions in plaintext to the world many privacy solutions use cryptographic methods vulnerable to quantum computing

any protocol that relies on elliptic curve cryptography will be cracked once quantum computers can run Shor's algorithm

you can't patch this later. if the cryptography used to encrypt your data is not post-quantum secure, it's vulnerable forever, no matter what upgrades come after